1. Field of the Invention
This invention relates to an apparatus for sensing the alignment marks of two objects when the two objects are to be aligned with each other, extracting a necessary signal component from each sensed signal, and composing the extracted signals to obtain a desirable signal.
2. Description of the Prior Art
During the patterning in the process of manufacturing semiconductors, a wafer and a mask must be aligned with high accuracy and recently, this is usually accomplished automatically. In the automatic aligning process, photoelectric detecting means are generally used to detect the position of an object. For example, in an apparatus of the prior art, the object is scanned by using a laser light as a light source and the light beams scattered from alignment mark patterns W and M on the wafer and mask shown, for example, in FIGS. 1A and 1B of the accompanying drawings are photoelectrically detected by a photodiode and, by utilizing the fact that the output signal thereof includes the information on the distance between the patterns W and M, the relative position of the wafer and mask is detected. Such aligning is accomplished by directing the mark patterns W and M detected in the described manner into a positional relation as shown in FIG. 1C. The mark patterns W and M and a pattern constituting an actual element are in a predetermined relation and therefore, if the mark patterns W and M are directed into a predetermined positional relation, the actual element patterns on the wafer and mask are properly aligned. If the positional relation between the mark patterns W and M is detected, the difference between that positional relation and the predetermined relation may be examined and a driving mechanism may be operated so that this difference becomes null. The aligning of the wafer and mask requires all the degree of two-dimensional freedom to be controlled and is usually accomplished by observing a plurality of locations on the wafer and mask.
The alignment marks depicted in FIGS. 1A and 1B are known from U.S. Pat. No. 4,167,677 of the present assignee.
On the other hand, when the coherent lights coming back from a set of marks on the wafer and mask are detected by a single photoelectric detector, the two lights interfere with each other to provide an unstable signal and this has led to the disadvantage that automatic aligning becomes impossible or the accuracy of aligning is aggravated.
In a specific example shown in FIG. 5 of U.S. Pat. No. 4,251,129 of the present assignee, the light signals from the wafer and mask are detected by different photoelectric detectors, wereby a stable signal is obtained. However, even if great improvements have been made by this apparatus, undesirable signals concerning the mark on another object to be detected may be present in the signals produced by the detectors. On the other hand, the peak signals of the obtained signals which correspond to the mark patterns W and M tend to be wide in their rising width, but the levels of the peak signals are not always higher than a predetermined level. Therefore, it is very difficult to sharpen the signals by determining a peak cut value, to obtain aligning information of high accuracy.